AH, TO be in Paris in the springtime. Strolling the tree-lined banks of the River Seine. Drinking café au lait while gazing out from the heights of Montmartre. Roaming underground amid corpses and raw sewage ...

It's true. Lurking beneath charming café-strewn boulevards, two bizarre museums beckon urban adventurers to subterranean labyrinths. If that interests you, consider ditching the sunglasses and taking a walk on the dark side in the City of Light.

The first attraction on the itinerary may cause you to cover your nostrils. Paris, with museums showcasing food, music, history, and some of the Western world's finest art, also has a museum dedicated to its effluent. The Paris Sewer Museum exposes a fascinating but fetid layer of urban infrastructure. After the Black Death decimated the population, the city labored to stem public health hazards in the ever expanding metropolis. It built its first underground sewer in 1370, but this prototype did little more than ferry untreated waste to the Seine. In 1850 engineer Eugène Belgrand designed the elaborate aqueducts that now extend for more than a thousand miles. Until 30 years ago, visitors could even take boat rides in the sewer canals. Definitely not something you'll find at Disneyland.

The sewers have been immortalized in a number of literary classics, though, a testament to their mysterious hold on popular imagination. In The Phantom of the Opera, the title character calls them home sweet home. Sections of Victor Hugo's Les Misérables read like a paean to the Parisian underground, such as Jean Valjean's heroic escape through a minefield of rats and quicksand.

The gargantuan sewer complex runs under every city street, and the museum illustrates a classic union of antiquated and contemporary technology. Workers known as égoutiers still pull clog-busting barges, or "flushing boats," through the main sewers to clean out that stubborn sediment. But you can also peer through metal grates and watch the active channels as they flow off to modern treatment plants. A museum highlight from the other end of the fashion spectrum is égoutier models in black rubber pants, surely the utmost in wastewater fetish wear. There's also a detailed exhibit on the history of the water cycle in Paris.

Every now and again the fragrance turns not so sweet, but it's mostly just a musty odor. If you're lucky, you might even happen on a furry little sewer dweller. And of course, you won't want to neglect the museum's best interactive feature – restrooms are conveniently located at the end of the tour.

Now let us move on from sewage to corpses. In the late 1700s Paris sought a lasting solution to its overcrowded and dangerously unsanitary graveyards. But with real estate at a premium, what to do with all those pesky bodies? Welcome to the Catacombes, another fantastical Parisian underworld. Situated in sprawling quarry tunnels dating back to the Romans, the Catacombes are the final resting place for almost six million individual human remains. Sixty-five feet below daylight, a sign painted above an entryway warns, "Stop! This is the empire of the dead," as if you've gotten off on the wrong floor of the Eiffel Tower.

Instead you'll enter a riveting maze of skeletons and sepulchers. The ossuary's dank stone corridors wind past stacks of skulls and cavernous rooms crammed with artfully organized bones. Remains were discreetly disinterred from cemeteries and moved en masse to these rambling tunnels, solicitously labeled with their graveyard of origin. A continuous black line at the apex of the passageway ceilings looks like Hansel and Gretel switched from bread crumbs to spray paint, but it turns out to be centuries of singed lantern smoke. Interspersed throughout the towering heaps of human decay are choice epitaphs about the fleetingness of life. Touché.

Ensconced in the catacomb tunnels are relics from key periods in French history. During World War II the French Resistance employed the tunnels to stage attacks, while the Germans fashioned an underground bunker. In a crypt somewhere lie the decapitated remains of Robespierre and Marie Antoinette, but good luck finding them among the masses. And in case you get any morbid ideas about souvenirs, be forewarned: guards search for unauthorized artifacts at the exit.

Astute urban spelunkers may have noted that both museums make up only a fraction of the Parisian underground network. Although they are closely guarded secrets, scores of unmarked entrances in railway lines and down manholes provide access to the many miles of dim and meandering tunnels. The Internet is full of reconnaissance reports by self-described cataphiles with pirated maps and contraband keys. Illicit explorations are so prevalent that special tunnel police officers patrol for after-hours tourists and the occasional rave.

In 2004 police stumbled on an enormous hidden amphitheater that housed a literally underground film club. Besides a projector, a cache of movies – including some obligatory film noir – a full-size screen, and a bar, the club had rigged up electricity and phone lines. Soon after discovery, the unknown proprietors surreptitiously returned to cut the utilities and leave a simple missive: "Do not try to find us." Beth Kohn is a San Francisco-based freelance writer and aspiring cataphile.

Trip planner

Going under Les Catacombes, Tues.-Sun., 10 a.m.-4 p.m., 1 Place Denfert-Rochereau. Metro: Denfert-Rochereau. $7. 01-43-22-47-63. Paris Sewer Museum (Musée des Égouts), Sat.-Wed., 11 a.m.-5 p.m. (Oct.-April until 4 p.m.; closed last three weeks of January), Pont de l'Alma, opposite 93 Quai d'Orsay. Metro: Alma-Marceau. $4. 01-53-68-27-81.